Target detection by doppler shift

ABSTRACT

Detection of moving targets by detection of the doppler shift of radiation scattered or reflected by the target when illuminated by such radiation, is liable to give a false target indication if an interfering signal is received which has a frequency difference from the transmitted radiation lying in the range of the expected doppler shift. The problem is found in intruder alarm systems which use doppler shift detection. To mitigate the problem, the transmitted radiation is frequency modulated, by noise or by an audio tone, so that the scattered or reflected radiation has a coherence with the transmitted radiation but the interfering signal does not. The principle is applicable to both electromagnetic and pressure-wave radiation. In one intruder alarm system two separately operating, doppler shift detector units are coupled together to give a warning signal only when both indicate an intruder. Application of the above principle to one or both units prevents them giving false alarms due to mutual interference.

United States Patent J inman TARGET DETECTION BY DOPPLER SHIFT MichaelJinman, Edinburgh, Scotland Filed: Mar. 22, 1971 Appl. No.: 126,744

Inventor:

Assignee:

[451 Aug. 21, 1973 Primary Examiner-Malcolm F. Hubler Atlomey-Sughrue,Rothwell, Mion, Zinn & Macpeak [5 7] ABSTRACT Detection of movingtargets by detection of the doppler shift of radiation scattered orreflected by the target when illuminated by such radiation, is liable togive a false target indication if an interfering signal is receivedwhich has a frequency difference from the transmitted radiation lying inthe range of the expected doppler [30] Foreign Appucafion Priority Datashift. The problem is found in intruder alarm systems Mar. 20, Great usedoppler detection To the problem, the transmitted radiation is frequencymodu- 343/73 340/258 343/5 PD lated, by noise or by an audio tone, sothat the scattered o reflected radiation has a coherence th tra [58]Fleld of Search 343/ 5 PD, mitted radiation but the interfering signaldoes not. The 340/258 A principle is applicable to both electromagneticand pressure-wave radiation. In one intruder alarm system ReferencesCited two separately operating, doppler shift detector units UNITEDSTATES PATENTS are coupled together to give a warning signal only when3,242,486 3/1966 Corbell 343 5 PD both indicate an immder- ApplicationOf the above 3,343,167 9/1967 Rademacher... 343/5 PD principle to one orboth units prevents them giving false 3,383,678 5/1968 Palmer 343/5 PDalarms due to mutual interference.

3,512,155 5/1970 Bloice 343/7.7 2,649,538 8/1953 Marlowe et al 343 5 PD13 Clams, 2 Drawing Figures y PAQ/LQ u/v/r AfA /AA -AE/8/AA K M/XEQ g? Q4 7c- T/P/I/VjM/TTE/P l -M 05C/A L A 70E m rze g a, 27 64 0 52 2.: ffl71 75/? i N055 flMfiA/fl/Df 547-4 j fl AV/EA/[J/A/' 6%6'0/7 -4J W 75570g TARGET DETECTION BY DOPPLER SHIFT This invention relates toapparatus for a system for and a method of target detection utilisingthe doppler shift of radiation reflected by or scattered by a targetwhich is illuminated by such radiation. The radiation may beelectromagnetic particularly in the microwave region or it may bepressure wave particularly at ultrasonic frequencies.

The invention will be described in relation to a radar apparatus but itwill be realised that the principles given are equally applicable topressure wave radiation.

The invention finds particular application in intruder alarm systems.Such systems may be used in the open air protecting a yard or other openspace where the propagation can be considered to be free-space, in whichcase the target moving radially from the radar apparatus and illuminatedthereby scatters radiation back to the apparatus which has a dopplershift component. The extent of shift is a function of radial velocitybut in an intruder alarm system it is not generally required to measurevelocity. Intruder alarm systems are, however, frequently used in caseswhere they are located in a warehouse or other enclosed space. In such aspace, especially where high frequencies are used of the order of 10,000MHz there is substantial reflection from walls and all permanent objectsin the space and the free space propagation concept does not apply. Dueto the multiple reflections there is set up a standing wave pattern ofelectromagnetic field in the surveyed space. Thus at any point in thefield a target can be regarded as having impinging on it radiationcomponents received from a number of directions other than directly fromthe radar apparatus. The movement of the target will cause dopplershifts in these components and the reflected or scattered radiationhaving this doppler shift will be received by the radar apparatus partdirect and part having itself been reflected by walls etc. Thus thesituation here is very complex but the resultant signal received at thereceiver will be referred to as the doppler shift signal notwithstandingthat it may be comprised of a number of components set up in the mannerdescribed.

In one form of intruder alarm a warning is given if there is detected areceived signal having three characteristics: a frequency differencefrom the transmitted frequency within a predetermined band (normally theactual measurement of target velocity is not required); a duration longenough to cause the alarm to respond; and an amplitude sufficient tocause operation of the alarm to provide a warning signal.

The frequency difference is conveniently detected by heterodyning(sometimes called mixing) the received echo signal with a portion of thetransmitter output and extracting the difference frequency signal. Theband within which frequency difference is detected may be specified by afilter and/or may be determined by the inherent frequency limitations ofthe receiver circuits and the detector. The term receiver pass-band willbe used to cover all these cases. A received signal need not becontinuously present to cause the alarm to respond if the receiverincludes an averaging or integrating circuit having a time constantenabling it to respond to a succession of short bursts of the echoduring the averaging or integration period. A target-indicative signalis produced if the resultant over the period reaches a given level. Thesignal amplitude required to cause the system to respond may bedetermined by the inherent sensitivity of the receiver; or a deliberateamplitude gate may be employed to ensure that only signals above acertain amplitude are efl'ective and to eliminate signals at or aboutthe noise level of the system.

A false warning of an intruder may be given if there is received aninterfering signal, not being a target echo and being unrelated to thetransmitted signal, which has a frequency such as to provide adifference frequency signal within the receiver pass-band and whichmeets the amplitude and duration requirements of the system so as tocause the warning signal to be given. This situation can arise where twoseparate intruder alarms employing doppler shift radar are in relativelyclose proximity even though each is outside the accepted surveillancerange of the other, it being assumed that the two radar systems operateon nominally the same or adjacent frequencies. The receiver of eachradar may receive directly the transmitted radiation of the other. Dueto factors such as drift in the transmitter frequencies these may have afrequency difference lying within the pass-band of each receiver for alength of time sufficient to provide a warning signal, each re- 7 ceivedsignal being of sufficient amplitude for this to occur even though thetwo alarms lie outside one anothers surveillance areas. The reason forthis is that the received echo from a target decreases in strength asthe fourth power of increasing range whereas the radiated field strengthreceived directly from a transmitter decreases as the square of therange. The interference in such cases is likely to be mutual.

It has been proposed that in order to improve and shape the coverage ofa surveyed space that more than one radar unit be used so that a warningis only given if the presence of an intruder is detected by all unitssimultaneously. The units may be coupled together through an AND-gate orthe like to this end. In such an arrangement each unit is likely toreceive a strong direct signal from the other, or at least one of theother units, and between the two. or any two units the interferenceproblem is likely to be mutual.

The severity of the interference problem in any particular case may beexpressed as the probability of effective interference occurring withina set period which may becalled the measurement period: the longer themeasurement period the higher the probability. In the case of intruderalarms the measurement period isvery long. It is therefore desirable totake steps to reduce this probability.

The present invention is based on the concept of so modulating thetransmitted radiation for target detection by means of doppler shift (asdefined hereinbefore) that a receiver responds only to receivedradiation which is due to radiation from its own transmitter. inparticular the transmitted radiation may be frequency modulated suchthat, although the instantaneous frequency of the radiation may be moreoften near the frequency of an interfering signal than would otherwisebe the case, a difference frequency within the receiver pass-band isproduced for too short periods to cause a target to be indicated.

In one aspect of the present invention, there is provided a method ofoperating a target detection apparatus in which a target is indicated bythe detection of a received signal having a frequency difference fromthe transmitted frequency by a value within a predetermined rangeand'lasting for a predetennined duration,

wherein the transmitted frequency is modulated in such manner that thereis a low probability of the difference frequency between the transmittedsignal frequency and the frequency of a received interfering signalbeing within said predetermined range for sufficient time to indicatethe presence of a target. Preferably the trans mitted frequency isfrequency modulated by a noise signal but an audio tone may be used.

In a further aspect of the invention there is provided a targetdetection apparatus comprising a transmitter, means for frequencymodulating said transmitter, a re ceiver for producing a signalrepresenting the difference frequency between the transmitted frequencyand the frequency of a received signal, means responsive to the presenceof a difference frequency-representing signal which represents adifference frequency within a predetermined range and lasts for apredetermined duration to provide a signal indicative of the presence ofa target, said frequency modulating means being arranged to modulate thetransmitted frequency in such manner that there is a low probability ofthe difference frequency between a received interfering signal and thetransmitted signal being within said predetermined range for sufficienttime to cause said target-indicative signal to be produced.

In a still further aspect the invention provides a target detectionsystem comprising two target sensing units, each unit comprising atransmitter, an aerial system for radiating the energy generated by thetransmitter and for receiving radiation, a receiver for the receiverradiation operable to produce a signal representing the differencefrequency between the transmitted signal and the frequency of a receivedsignal, and means responsive to the presence of a differencefrequencyrepresenting signal which represents a difference frequencywithin a predetermined range and lasts for a predetermined duration toprovide a signal indicative of the presence of a target; and at leastone of said units including means for frequency modulating thetransmitter frequency thereof in such manner that there is a lowprobability of the difference frequency between an interfering signalreceived at said one unit and the transmitted signal of said one unitbeing within said predetermined range for sufiflcient time to cause saidtarget-indicative signal to be produced; the aerial systems of said twounits being arranged to survey a common volume of space; and the targetdetection system further comprising means coupled to receive thetarget-indicative signals from said two sensing units to produce awarning signal only if target-sensing signals are generatedsimultaneously by said two sensing units.

Preferably the frequency modulation means in the apparatus or systemabove defined provides a noise or audio tone modulation of thetransmitted frequency.

The problem of interference signals may be greatly reduced by frequencymodulating the transmitter signals in such a manner that a receiverresponds only to the signals originating from its own transmitter.

It is also necessary to ensure that the modulating signals do notinteract to produce significant signals within the receiver pass-band,which may occur, for example, if the difference between the modulatingfrequencies of two transmitters lies in this pass-band. To this endnoise modulation, modulation by several randomly chosen frequencies, ormodulation by selected frequencies in predetermined channels, may beused.

As an example, frequency modulation of the transmitters of an'X-bandintruder alarm system by noise has reduced interference signal levels by30 db without affecting the other characteristics of the system.

The invention may be applied to range, velocity or proximity measurementby means of radar or pressure wave apparatus operating with dopplershift.

The two embodiments of the invention will now be described withreference to the accompanying drawings in which:

FIG. 1 shows a block diagram of an intruder alarm system using a dopplershift radar; and

FIG. 2 shows a multiple radar unit intruder alarm system.

In FIG. 1 the intruder alarm system comprises a radar apparatus 10having a CW transmitter 12 including an oscillator 14 for controllingthe transmitter frequency. The oscillator may provide the outputdirectly or through an amplifier. The oscillator has connected thereto anoise or tone source 18. The oscillator is preferably a Gunn diodeoperating in the X-band (10,000 MHZ). The oscillator frequency ismodulated in accordance with the amplitude of the noise or tone signal.Amplitude modulating the Gunn diode operating current causes a frequencymodulation of the microwave output therefrom. When noise is used formodulation the white noise from source 18 may be passed throughfiltering and clipping devices 20 and 22 respectively to limit theamplitude and frequency range of the noise modulation signal and therebylimit the transmitted frequency deviation. The reasons for so doing areexplained below. An aerial 24, for example a horn, is coupled to receivethe transmitter output and to illuminate a space to be surveyed.

,The radar apparatus further has a receiver 30 fed from an aerial 32,for example a horn, and including a mixer 34 to which are appliedreceived signals and a proportion of the transmitter output, as forexample, by a directional coupler 36 or a simple waveguide sectioncoupling the mouths of the horns; or simply by leakage of energy fromthe transmitter to the receiver. The output of the mixer 34 feeds a lowpass or band-pass filter 38 and the filter output is applied to adetector 40 which includes a time-delay circuit 44 and provides awarning signal W if there is an output from the filter present for atleast a predetennined period of time. The detector may further includean amplitude gate 42 so that only mixer output signals above a certainamplitude can result in a warning signal W.

Mention has already been made of the detection of signals which are notnecessarily continuous and to this end the detector preferably includesan integrating or averaging circuit as the delay circuit 44 whereby awarning signal is produced if some minimum level of output is achievedover the specified period set by the time constant of the circuit. Inparticular the detector circuit described in our copending applicationSer. No. 5,5301 may be used.

The filter 38 may have an upper cutoff frequency selected to avoid thealarm being triggered by the passage of birds or fast moving insectsthrough the surveyed space. Birds generally fly sufficiently quickly togive a higher doppler shift than would be expected from the movement ofa human being in the surveyed space. Even if the filter 38 is omitted,the receiver will have a restricted bandwidth iimiting the range offrequencies which can be passed to the detector. The filter may alsohave a lower frequency cut-off. This is particularly useful in pressurewave apparatus to avoid the effects of air movements.

The detection of moving targets by doppler shift needs no descriptionhere. The frequency modulation of the transmitted signal does not affectthe detection of doppler shift provided that certain conditions arefulfilled. These conditions are discussed below.

If, assuming for the moment the source 18 is switched off, there isreceived an external or interfering signal at a frequency which, whenheterodyned with unmodulated transmitter frequency in the mixer 34,produces a difference frequency within the receiver pass-band and whichis of sufficient amplitude to actuate the detector, then a false warningsignal will be given.

The probability of such interference may be reduced by modulating thefrequency of the transmitter in such a manner that whilst coincidence ornear coincidence of the transmitter frequency and the interfering signalfrequency is more likely to occur, the time during which the differencefrequency lies within the receiver pass-band on each occasion is tooshort for a warning signal to be given. In general where the radartransmitter signal and the interfering signal are not related, it can beshown that the optimum modulating signal is a noise signal through otherforms of modulation such as a tone all produce considerable benefit. Ifthe instantaneous transmitter frequency is substantially constant overthe period for the radiation to travel to the target and back, thedetected doppler shift signal will not be affected by the appliedfrequency modulation. Thus there will be a coherence between thetransmitted and received radiation pertaining to the unit but no suchcoherence with an external signal.

FIG. 2 shows an example of a multiple unit intruder alarm using tworadar units 50, 52 the receiver of each of which will recieve a strongsignal directly from the transmitter of the other. The two units areidentical and of the kind shown in FIG. 1. The units 50, 52 haveresponse patterns 51, 53 respectively and the common surveyed space isthat where the patterns overlap. The detector output of each radar unitis connected to a coincidence gate 54 from which a warning signal isobtained upon simultaneous sensing of a target by the two units 50, 52.If in the absence of modulation of the transmitter frequencies, thelatter have a difference frequency within the receiver pass-bands eachunit will detect an apparent moving target and the resultingsimultaneous detector outputs will give a false warning signal.

Noise or tone modulation of the transmitter frequencies in the mannerdescribed will give protection against such mutual interference. Anotherway of looking at this is that there will be a coherence between thetransmitted and received radiation pertaining to one unit but no suchcoherence as regards received radiation from the other unit or othersource. Obviously where units are used in close proximity, themodulation frequencies are chosen to give the least likelihood of mutualinterference.

In fact only one transmitter need be frequency modulated and in multipleunit alarm systems using three or more units not all the transmittersneed be frequency modulated to give the required protection. However,from a manufacturing point of view it is better to manufacture identicalunits, each being tone or noise modulated.

Apart from intruder alarm systems the invention is applicable to systemsin which it is desired to actually. measure the proximity, range orvelocity of a moving target using CW or pulse transmission.

The optimum frequency deviation and band width of the modulation dependson the characteristics of the particular system. Limits are set by thepermitted frequency band limits in many systems, and by the requirementthat the maximum frequency of the noise modulation or the tone frequencybe much less than the reciprocal of the system transmission path time.(i.e., when detecting an echo at maximum range the transmitter frequencyat the time the echo is received should not have departed significantlyfrom the frequency of the signal which gave rise to the echo).

Despite these restrictions the usefulness of frequency modulation inreducing the probability of mutual interference, is considerable. As anexample calculation indicates a reduction in this probability by 1,000times in a particular intruder alarm system: in experiment, it has beenfound virtually impossible to induce interference in a noise modulatedsystem, but relatively easy to do so in an unmodulated system. A tonemodulation system may be less effective than noise but is much simplerand cheaper and in practice gives good results.

I claim: 1. Apparatus for detection of a moving target comprising:

a transmitter for producing output signals, and a receiver for receivingtarget echo signals and interfering signals, said receiver including amixer responsive to a portion of the transmitter output signals andtarget echo signals for deriving therefrom Doppler shift signalsresulting from the motion of the target illuminated by radiatedtransmitter output signals, means for blocking mixer output signalsoutside a prescribed frequency passband, and time duration detectormeans responsive to mixer output signals within said passband forproducing a target indicative signal upon receipt of a plurality ofcycles of said mixer output signals within a predetermined timeinterval,

wherein said transmitter includes means for frequency modulating saidtransmitter output frequency at an average frequency sweep ratesufficiently high that, upon reception of an interfering signal having afrequency range within the frequencydeviation range of said transmitteroutput signal, the time for which the difference frequency therebetweenlies within said prescribed passband is too short to allow a significantmixer output signal to be derived in response to the interfering signal.

2. Apparatus as claimed in claim 1 wherein said frequency modulatingmeans comprises a noise source.

3. Apparatus as claimed in claim 2 wherein said frequency modulatingmeans includes means for limiting the frequency deviation of thetransmitted frequency.

4. Apparatus as claimed in claim 1 wherein said frequency modulatingmeans comprises a source of a repetitive waveform in the audio frequencyrange.

5. Apparatus as claimed in claim 1 wherein said transmitter operates inthe microwave frequency region.

6. Apparatus as claimed in claim 5 wherein said transmitter comprises aGunn diode oscillator.

7. Apparatus as claimed in claim 6 wherein said frequency modulatormeans is arranged to provide an amplitude modulated operating currentfor said Gunn diode oscillator whereby the latter is frequencymodulated.

8. Apparatus as claimed in claim 1 wherein said receiver furtherincludes an amplitude gate means for allowing only Doppler shift signalsabove a predetermined level to cause a target-indicative signal to beproduced.

9. Apparatus as claimed in claim ll wherein said time duration detectorincludes a circuit means for averaging the mixer output signals over apreselected period of time.

10. A system for detecting a moving target comprising a first movingtarget detection apparatus as claimed in claim 1, and a second movingtarget detection apparatus which comprises a transmitter, producing anoutput signal anda receiver having a mixer responsive to a portion ofthe transmitter output signal and to target echo signals to derivetherefrom Doppler shift signals resulting from the motion of a targetilluminated by the radiated output signal from the transmitter, andmeans coupled to said receiver mixer and responsive to Doppler shiftsignals lying within a prescribed frequency band to produce a targetindicative signal upon detection of a plurality of cycles of saidprescribed Doppler shift signal, wherein said first and second targetdetection apparatus are disposed relative to one another such that, inoperation, each receives some radiated transmitter output signal fromthe other.

11. A target detection system as claimed in claim 10, wherein eachapparatus comprises a respective aerial system for radiating saidtransmitter output signal and for receiving radiation to be coupled tothe associated receiver mixer, said aerial systems being arranged tosurvey a common volume of space.

12. A target detection system as claimed in claim 11, further comprisingmeans coupled to the respective Doppler shift signal responsive means ofsaid first and second apparatus and responsive to said targetindicativesignals therefrom to produce a warning signal only if said targetindicative signals are generated simultaneously.

13. A target detection system as claimed in claim 10, wherein saidsecond apparatus further includes means operable to frequency modulatesaid transmitter output signal thereof at an average frequency sweeprate sufficiently high that, upon reception of an interfering signalhaving a frequency within the frequency deviation range of saidtransmitter output signal, the time for which the difference frequencybetween the interfering signal frequency and the instantaneoustransmitter output signal frequency lies within said prescribedfrequency band is too short to allow an apparent Doppler shift signal tobe derived from said receiver mixer of said second apparatus, andwherein the respective frequency modulation means of said first andsecond apparatus are operable at different modulating frequencies.

1. Apparatus for detection of a moving target comprising: a transmitterfor producing output signals, and a receiver for receiving target echosignals and interfering signals, said receiver including a mixerresponsive to a portion of the transmitter output signals and targetecho signals for deriving therefrom Doppler shift signals resulting fromthe motion of the target illuminated by radiated transmitter outputsignals, means for blocking mixer output signals outside a prescribedfrequency passband, and time duration detector means responsive to mixeroutput signals within said passband for producing a target indicativesignal upon receipt of a plurality of cycles of said mixer outputsignals within a predetermined time interval, wherein said transmitterincludes means for frequency modulating said transmitter outputfrequency at an average frequency sweep rate sufficiently high that,upon reception of an interfering signal having a frequency range withinthe frequency deviation range of said transmitter output signal, thetime for which the difference frequency therebetween lies within saidprescribed passband is too short to allow a significant mixer outputsignal to be derived in response to the interfering signal.
 2. Apparatusas claimed in claim 1 wherein said frequency modulating means comprisesa noise source.
 3. Apparatus as claimed in claim 2 wherein saidfrequency modulating means includes means for limiting the frequencydeviation of the transmitted frequency.
 4. Apparatus as claimed in claim1 wherein said frequency modulating means comprises a source of arepetitive waveform in the audio frequency range.
 5. Apparatus asclaimed in claim 1 wherein said transmitter operates in the microwavefrequency region.
 6. Apparatus as claimed in claim 5 wherein saidtransmitter comprises a Gunn diode oscillator.
 7. Apparatus as claimedin claim 6 wherein said frequency modulator means is arranged to providean amplitude modulated operating current for said Gunn diode oscillatorwhereby the latter is frequency modulated.
 8. Apparatus as claimed inclaim 1 wherein said receiver further includes an amplitude gate meansfor allowing only Doppler shift signals above a predetermined level tocause a target-indicative signal to be produced.
 9. Apparatus as claimedin claim 1 wherein said time duration detector includes a circuit meansfor averaging the mixer output signals over a preselected period oftime.
 10. A system for detecting a moving target comprising a firstmoving target detection apparatus as claimed in claim 1, and a secondmoving target detection apparatus which comprises a transmitter,producing an output signal and a receiver having a mixer responsive to aportion of the transmitter output signal and to target echo signals toderive therefrom Doppler shift signals resulting from the motion of atarget illuminated by the radiated output signal from the transmitter,and means coupled to said receiver mixer and responsive to Doppler shiftsignals lying within a prescribed frequency band to produce a targetindicative signal upon detection of a plurality of cycles of saidprescribed Doppler shift signal, wherein said first and second targetdetection apparatus are disposed relative to one another such that, inoperation, each receives some radiated transmitter output signal fromthe other.
 11. A target detection system as claimed in claim 10, whereineach apparatus comprises a respective aerial system for radiating saidtransmitter output signal and for reCeiving radiation to be coupled tothe associated receiver mixer, said aerial systems being arranged tosurvey a common volume of space.
 12. A target detection system asclaimed in claim 11, further comprising means coupled to the respectiveDoppler shift signal responsive means of said first and second apparatusand responsive to said target-indicative signals therefrom to produce awarning signal only if said target indicative signals are generatedsimultaneously.
 13. A target detection system as claimed in claim 10,wherein said second apparatus further includes means operable tofrequency modulate said transmitter output signal thereof at an averagefrequency sweep rate sufficiently high that, upon reception of aninterfering signal having a frequency within the frequency deviationrange of said transmitter output signal, the time for which thedifference frequency between the interfering signal frequency and theinstantaneous transmitter output signal frequency lies within saidprescribed frequency band is too short to allow an apparent Dopplershift signal to be derived from said receiver mixer of said secondapparatus, and wherein the respective frequency modulation means of saidfirst and second apparatus are operable at different modulatingfrequencies.